DESCRIPTION: (Applicant's Description) The proposed resource will provide positron imaging of small animals for NCI funded investigators at the University of Washington. Two small animal machines will be constructed and supported for imaging studies. These systems will implement detector and electronics technology that has been under development in our laboratory for the past six years. The recent availability of new, multi-cathode photomultiplier tubes PMTs) and a reliable source of Lutetium Oxyorthosilicate (LSO) crystals has now made it possible to build these systems for general use. The first system is optimized for imaging mice and small rats. It will have a 12 cm ring diameter, providing an 8 cm transverse field-of-view (FOV). This system will utilize 0.8x0.8x6 mm LSO crystals to provide a limiting resolution of approximately 1 mm. The system will ultimately support three rings of modules spanning 6.7 cm axially. The system will first be built with a single ring of modules and made available for use by the end of the first year of finding. During the second year, the additional two rings will be added. A unique feature of this system is that the second and third rings are movable in the axial direction. In this way, the system can be operated with all three module rings adjacent or disjoint (e.g., two adjacent over the head and one over the heart so that an input function can be measured by imaging the ventricle). The second system will have a 20 cm ring diameter, providing a 15 cm FOV and is designed to image large rats, rabbits, and small primate heads. This is a more complex system with 2x2x20 mm LSO crystals that uses depth-of-interaction information to preserve radial resolution. The complete system will consist of 24 rings of crystals (8 rings in each of three module rings). A DOI system with a single ring of modules will be complete by the beginning of year 3. The remaining two rings will be constructed and installed during years 3 to 5; The last ring will be moveable in the axial direction. The resource personnel will provide support for imaging (technologist time, calibrations, image reconstruction, and assistance with image analysis). Animal support and radiopharmaceuticals will be provided by existing cost centers within the University of Washington Department of Radiology.